When calculating the damage of 1 person it is pretty much 100% accurate. In general the current parser (ahem, which I'm not using of course!!, /wink) is within a 2% error margins that get corrected when uploaded to fflogs (that's why the dps on fflogs is usually lower than *ahem* said certain not-allowed program*cough*).

Also GMs I love you too, don't ban me for using the bad p-word! (not as much as Yoshida-San of course).